Antibiosis, the production of a substance by an organism to influence the activities of another organism, is a very common phenomenon in the marine environment. We propose to examine examples of antibiosis between marine organisms in order to discover new compounds having the selective activity required for pharmaceutical application as antibiotics. We have recently determined the structures of several compounds having antibiotic activity from marine bacteria, sponges, molluscs and algae. We have also synthesized aeroplysinin-1, an antibiotic from a sponge. Over 10% of the organisms which have been screened have shown antibiotic activity in vitro. We will continue to isolate and identify the active constituents from these organisms. Structural elucidation will be accomplished by spectroscopic means or by single crystal X-ray analysis. Active compounds will be submitted for pharmaceutical evaluation. Potentially useful compounds will be synthesized to provide material for further studies.